1. Field of the Invention
The present invention is related to a process for bringing together two suspension layers, such that a first aqueous pulp suspension layer is brought together with at least one additional suspension layer. The two suspension layers are transported on moving layer supports to a wedge-shaped gap and drained. The present invention is also related to a decker in which the suspension layers are brought together.
2. Discussion of Background Information
Processes of the type generally discussed above, are used, e.g., in paper production, and, in particular, in the preparation of paper pulp for such production. These processes are generally utilized as intermediate steps in the overall preparation process. A large part of the preparation process is performed in aqueous pulp suspensions, i.e., the paper fibers can be transported in pipes and pumps after being mixed with water. For various reasons, it may be necessary to drain the pulp suspension, e.g., to decker it, for subsequent processes or to wash out undesirable particles. The draining of the suspension often takes place on wire or water-permeable cylinders. In this manner, the water runs off through the openings and the paper fibers that remain behind are deckered.
Such processes have long been known, with the result that there is a large number of different, i.e., more or less complex, processes by which the pulp suspension is drained. In the course of such processes, it is often desired to combine, i.e., to bring together flatly, already drained suspension layers with another suspension layer. In these processes, a mixing of the layers brought together, i.e., an exchange of material between the layers on the contact surfaces, should be avoided. Efforts have been made to avoid mixing by bringing the two layers to roughly a same speed before placing or brining them together. However, it is still not always possible to avoid damaging eddies.
The present invention provides a process in which it is possible to bring together suspension layers in the manner that largely avoids a mixing between the layers. Further, in special cases, the process may provide for controlled, limited mixing.
In the present invention, the suspension layers are guided on their sides or surfaces adapted for contacting each other, before, and until, being brought together, by guide surfaces that are one of stationary and very limitedly moving. The guide surfaces include contours that are arranged to run substantially parallel to the layer supports.
In the process according to the present invention, two suspension layers are arranged to slide along on the guide surfaces immediately before being brought together. A speed profile which develops has, considering the thickness of the layers, its smallest value in the immediate vicinity of the guide surfaces and its highest value in the vicinity of the layer supports. At a line at which the two layers first touch, the contacting surfaces of the suspension layers have a relatively low speed compared to their surroundings and move virtually parallel to each other. As is known, a laminar aqueous sublayer forms between a moving pulp suspension layer and a guide surface. In this manner, the operating conditions result such that a mixing between the two layers does not occur or is at least negligibly small.
The process according to the present invention finds an important application, particularly when the second suspension layer has a substantially lower solid content than the first pulp suspension layer. Such conditions may be present when filtrate, which contains solid particles which are to be reintroduced into the pulp suspension, is to be added to an already deckered pulp suspension. These solid particles may have been drained off upstream with the filtrate recovered there. If these solid particles are reintroduced into the pulp suspension using the process according to present invention and are drained through the pulp suspension layer, i.e., not in a direction toward a filter-side layer support, the solid particles directed into the filtrate are caught in the pulps suspension, which acts as a filter layer. Further embodiments are also conceivable in which, instead of filtrate and pre-drained pulp suspension, two pulp suspensions are brought together. In such a situation, it may be advantageous that the layer guided on the permeable layer support be lower in accepts or acceptable solid particles than the other layer (or layers). The layers thus brought together may be later withdrawn, e.g., after they have been further drained, and mechanically destroyed, i.e., torn apart again and often even rediluted. Thus, in these processes, formation of a paper web is not achieved.
Accordingly, the present invention is directed to a process for brining together a first aqueous pulp suspension layer with at least one additional suspension layer that includes moving the first pulp suspension layer and the at least one additional suspension layer on moving layer supports, and guiding sides of the first pulp suspension layer and the at least one additional suspension layer that are adapted to contact each other on guiding surfaces that are one of stationary and limitedly moving. The guide surfaces include contoured surfaces arranged to run substantially parallel to the layer supports. The process further includes applying the at least one additional suspension layer onto the first pulp suspension layer at a wedge-shaped gap, and draining the first pulp suspension layer and the at least one additional suspension layer.
In accordance with another feature of the present invention, the layer support for the first pulp suspension layer is water-permeable and the layer support for the second suspension layer is water-impermeable.
In accordance with another feature of the present invention, the process further includes moving the first pulp suspension and the second suspension layer at a substantially same speed in a region in which the first pulp suspension layer and the second suspension layer are brought together. Further, the substantially same speed is within a tolerance of approximately =10%.
In accordance with still another feature of the present invention, the first pulp suspension layer, at the time the second suspension layer is applied, has an average solid content of at most approximately 10%. Further, the first pulp suspension layer, at the time of the second suspension layer is applied, has an average solid content of at most approximately 4%.
In accordance with still another feature of the present invention, the second suspension layer has a solids content which is at most approximately 20% of a solid content of the first pulp suspension layer.
In accordance with a further feature of the present invention, the process further includes deckering the first pulp suspension layer prior to the wedge-shaped gap, accumulating filtrate from the deckering of the first pulp suspension layer, and applying the accumulated filtrate to a supply for the second suspension layer.
In accordance with a still further feature of the present invention, the process further includes deckering the first pulp suspension layer, accumulating a plurality of filtrate fractions, and using the first accumulated filtrate fraction as an additional suspension layer. Further, the additional suspension layer is composed of a pulp suspension having an accepts content that is greater than that of the first pulp suspension layer.
In accordance with another feature of the present invention, after draining, the process further includes removing the suspension layers from the layer supports, and mechanically destroying the removed layers.
In accordance with still another feature of the present invention, the guide surfaces have polished surfaces.
In accordance with still another feature of the present invention, the guide surfaces have friction-reducing surfaces.
In accordance with a further feature of the present invention, the guide surfaces have water-repellent surfaces.
In accordance with a still further feature of the present invention, the two guide surfaces are joined together in the wedge-shaped gap to form an angle of at most approximately 10.
In accordance with a further feature of the present invention, the moving layer supports are composed of a water-impermeable cylinder and an endless wire partially surrounding the water-impermeable cylinder.
The present invention is also directed to a decker to bring two suspension layers together that includes at least one rotating cylinder, and an endless wire guided around at least a portion of an outer circumference of the at least one rotating cylinder. The at least one rotating cylinder and the endless wire form a wedge-shaped gap. The decker also includes a first suspension introducing device for introducing a first suspension layer into the wedge-shaped gap, a displacer arranged upstream from the wedge-shaped gap that includes first and second guide surfaces, in which the first guide surface is arranged to be substantially parallel to a portion of the outer circumference of the at least one rotating cylinder adjacent thereto and the second guide surface is arranged to be substantially parallel to a portion of the endless wire adjacent thereto. At least one second suspension introducing device for introducing at least one second suspension layer into the wedge-shaped gap is also provided. The at least one second suspension introducing device is positioned to introduce the at least one second suspension layer upstream of the wedge-shaped gap and between the at least one rotating cylinder and the displacer.
In accordance with another feature of the present invention, the at least one rotating cylinder includes a jacket that is smooth on its circumference.
In accordance with still another feature of the present invention, the second guide surface has a radius of curvature of at least approximately 1 m. Further, the second guide surface has a radius of curvature of at most approximately 10 m.
In accordance with a further feature of the present invention, the first guide surface extends to be adjacent to a portion of the circumference that subtends an angle of at least approximately 15xc2x0.
In accordance with another feature of the present invention, at least one of the first and the at least one second suspension introducing devices includes a flow box. The flow box may have a broad stream nozzle.
In accordance with a further feature of the present invention, the first suspension introducing device is composed of a flow box arranged to introduce the first suspension layer on the endless wire and upstream from the displacer.
In accordance with a still further feature of the present invention, the at least one rotating cylinder is water-impermeable.
In accordance with still another feature of the present invention, the at least one rotating cylinder includes a first and a second rotating cylinder, such that the first rotating cylinder has a diameter greater than the second rotating cylinder, and the endless wire being guided around the first and the second rotating cylinder. The at least one second suspension layer is introduced between the second rotating cylinder and the displacer, and the first suspension layer is introduced between the first rotating cylinder and the wire.
In accordance with another feature of the present invention, the at least one rotating cylinder includes a first and a second rotating cylinder, such that the first and the second rotating cylinder have a substantially same diameter, and the endless wire is guided around the first and the second rotating cylinder. Each of the first and second rotating cylinder includes a displacer and an associated first and at least one second suspension introducing device.
The present invention is also directed to a process for stacking at least two suspension layers in a decker that includes a rotating cylinder, a wire guided around at least a circumferential portion of the rotating cylinder, and a displacer, positioned between the rotating cylinder and the wire, that includes first and second guide surfaces arranged substantially parallel to adjacent portions of the wire and the rotating cylinder, respectively. The process includes forming a first suspension layer on the wire, forming a second suspension layer between the rotating cylinder and the second guide surfaces, applying the second suspension layer to the first suspension layer downstream of the displacer to form a two layer suspension layer, and draining the two layer suspension layer.
In accordance with another feature of the present invention, the process further including guiding the first suspension layer between the wire and the first guide surface. Further, the process includes moving a surface of the first suspension layer adjacent to the wire at a greater velocity than a surface of the first suspension layer adjacent to the first guide surface. Still further, the process includes moving a surface of the second suspension layer adjacent to the rotating cylinder at a greater velocity than a surface of second suspension layer adjacent to the second guide surface. The first and second guide surfaces are mounted to be one of stationary and for only limited movement.
In accordance with still another feature of the present invention, the process further includes moving a surface of the second suspension layer adjacent to the rotating cylinder at a greater velocity than a surface of second suspension layer adjacent to the second guide surface.
In accordance with a further feature of the present invention, the guide surfaces are stationary surfaces.
In accordance with another feature of the present invention, the guide surfaces are mounted for only limited movement.
In accordance with still another feature of the present invention, the process further including draining the first suspension layer upstream of the displacer, guiding the first suspension between the wire and the first guide surface, and draining the first suspension between the wire and the first guide surface. Further, the process includes accumulating a first filtrate from the draining of the first suspension layer upstream of the displacer and between the wire and the first guide device, and applying the first filtrate to a supply for the second suspension layer. Further still, the process includes accumulating at least one second filtrate from the draining of the two layer suspension layer, and disposing of the at least one second filtrate.
In accordance with yet another feature of the present invention, the process further includes breaking down the drained two layer suspension layer in a macerator device.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.